1. Field of the Invention
The subject invention relates to a light emitting assembly of the type including light emitting diodes (L.E.D.s), and more particularly, efficient and full cutoff of light emissions.
2. Description of the Prior Art
Municipal or street light assemblies often generate spurious or scattered light emissions, which wastes usable energy. The scattered light also creates haze in the atmosphere, which obscures celestial objects and interferes with astronomical observations. Increased awareness of light pollution has created a demand for light assemblies achieving “full cutoff” or the “dark skies compliant” in the municipal and commercial lighting fields. Light assemblies meeting this criteria restrict or eliminate all light emissions above the horizon to reduce interference with astronomical observations. Full cutoff light assemblies also improve drivers' visual acuity by increasing contrast and reducing glare. In other words, light emitted by the light assemblies is directed onto the street rather than into drivers' eyes.
The U.S. Illumination Engineering Society has developed specifications for such a “full cutoff” designation. To meet the specification, the amount of light above eighty-five (85) degrees, i.e. upward light, emitting from the light assembly must be less than 1.5% of the total light flux of the light assembly, measured in lumens. Such a rigorous specification is difficult to achieve with conventional high intensity discharge (HID) single point light sources such as mercury, metal halide, or high pressure sodium lamps, due to geometric limitations. Specifically, the requirements for wide, non-scattered and uniform illumination, and the need to cut off light 5 degrees below the horizon, are difficult to reconcile in practical light assemblies, which typically include prismatic lenses that scatter light, unless the prismatic lenses are replaced with a relatively sophisticated reflector and aperture. An example of such an assembly is disclosed in the U.S. Pat. No. 7,244,050 Summerford et. al. The Summerford '050 patent discloses an HID light assembly including two sophisticated reflectors within a single light shield for achieving full cutoff.
In addition to achieving efficient and full cutoff light emissions, municipalities and commercial entities desire to replace HID street lamps with properly designed L.E.D. light assemblies. An example of such an assembly is disclosed in the U.S. Pat. No. 5,857,767 to the present inventor, Peter A. Hochstein, which is directed to effective thermal management. The Hochstein '767 patent discloses a light assembly including plurality of light emitting diodes disposed on a heat sink including a plurality of fins designed to enhance convective cooling. Proven metrics indicate that at least a fifty percent (50%) energy savings is possible due to the far greater service life that L.E.D. light assemblies offer.
At this time, and in the foreseeable future, L.E.D. light assemblies that are suitable replacements for conventional HID light sources, such as the assembly disclosed in the Hochstein '767 patent, contain a large number of L.E.D.s. These light assemblies are driven in series and/or parallel circuits to optimize their efficiency and generally occupy a much larger light emitting area than the HID light assemblies they replace. For example, while a 400 Watt HID light assembly might occupy an effective radiating area of a few square centimeters, an equivalent L.E.D. light assembly would present a source of several hundred square centimeters.
Obviously, such a distributed source is much more difficult to model optically, and to date it has not lent itself to effective, sharp cutoff beam shaping. With such relatively large and distributed source L.E.D. light assemblies, the simple expedient of using a single perimeter light shield to block high angle light, like those used for HID lamps, will not work. Each L.E.D. light source represents a unique geometry to the light shield. If a single light shield is used for a large number of L.E.D.s, light emitting from the L.E.D.s is scattered in undesired directions. If the entire light assembly is canted with respect to the horizon, the single light shield will be even more ineffective in controlling undesired scattered light. However, such canting or angular aiming of the entire light assembly is often required in order to properly cover the roadway surface with even illumination.
There remains a great need for an L.E.D. light assembly which achieves the full cutoff designation and prevents undesired scattered light to improve energy efficiency.